I just posted on a different post about a thump sensation when i ran across this post.

It seems to be an occasional issue but I noticed a slight thump sensation at the same time it is happening. I usually notice a drop to 500rpm then jump to 800-1000 rpm. If I sit at a stop for any length of time I notice continued rpm change usually in the same range and can feel the engine rpm change mostly when it drops off and almost feels like it is missing. On a few occasions noticed that I have let off the brake at the same time as the increase in RPM and the truck will lunge forward slightly. My wife has noticed it a few times also and has said something about it. Now have almost 7500mi and going to talk with the Service Dept when I go in for the 8000mi service.